This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2002-173777, filed on Jun. 14, 2002, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a ferroelectric capacitor having a crystalline thin-film of complex oxide formed by an MOCVD (Metal Organic Chemical Vapor Deposition) method and a method for producing thereof, and a ferroelectric memory comprising the ferroelectric capacitor.
2. Description of the Related Art
Conventionally, the structure of ferroelectric capacitors has been a planar-type. However, it is switching over to stack-type as capacitor integrity increases. A ferroelectric layer in a ferroelectric capacitor should be excellent in step coverage for realization of stereoscopic structure of the capacitor and should be high-density crystals for high ferroelectricity upon micronization of the capacitor. Therefore, the ferroelectric layer is conventionally formed not by the sol.gel method or sputtering method, but by the MOCVD method. Specifically, a ferroelectric layer formed of PZT is formed on one of a pair of electrodes of a noble metal such as Pt, Ir and the like or an electrically conductive oxide such as IrOx and the like by the MOCVD method and the other electrode is formed on the ferroelectric layer.
However, when the ferroelectric layer is formed of PZT by an MOCVD method, Pb in the ferroelectric layer of PZT may react with Pt of the one of a pair of electrodes material, whereby Ptpbx is formed and the surface becomes coarse. Therefore, Pt cannot be used as a material for one of a pair of electrodes. In addition, electrically conductive oxides such as IrOx commonly used as a material for one of a pair of electrodes cannot be used since they may be reduced during the formation of the PZT layer.
For these reasons, recently, Ir is used as a material for one of a pair of electrodes when forming a ferroelectric layer comprising PZT by the MOCVD method. An organic metal as a raw material is mixed with an oxidizing gas and is sprayed onto a wafer heated to 400xc2x0 C. to 700xc2x0 C., whereby the organic metal is thermally decomposed to form a ferroelectric layer comprising PZT on the one of a pair of electrodes comprising Ir. Then, the other electrode layer is formed by comprising IrOx on the ferroelectric layer comprising PZT in order to form a ferroelectric capacitor.
However, the ferroelectric capacitor thus obtained has problems of being inferior in fatigue properties and imprint properties. Therefore, there are needed a high-quality ferroelectric capacitor without such problems and an effective method for manufacturing a ferroelectric capacitor.
An object of the present invention is to provide a high-quality ferroelectric capacitor which is excellent in fatigue properties and imprint properties and an effective method for manufacturing it, and a high-quality ferroelectric memory comprising the ferroelectric capacitor. A ferroelectric capacitor according to the present invention comprises a ferroelectric layer formed by a MOCVD (Metal Organic Chemical Vapor Deposition) method, and a pair of electrodes contacting with the ferroelectric layer, wherein the ferroelectric layer has one of carbon and carbon atoms of 5xc3x971018 cmxe2x88x923 or less.
Since the amount of carbon atoms in the ferroelectric layer is small, the ferroelectric capacitor is excellent in fatigue properties and imprint properties.
A ferroelectric memory according to the present invention comprises a ferroelectric capacitor according to the present invention. Since the amount of carbon atoms in the ferroelectric layer is small, this memory is excellent in fatigue properties and imprint properties.
The first aspect of a process for manufacturing a ferroelectric capacitor comprises the steps of: forming a ferroelectric layer on one of a pair of electrodes; heating the ferroelectric layer at a temperature higher than a temperature for forming the ferroelectric layer to control carbon atoms of the ferroelectric layer to be 5xc3x971018 cmxe2x88x923 or less, and forming the other electrode on the ferroelectric layer. According to this process, carbon and hydrogen in the ferroelectric layer after forming a ferroelectric layer are released in vapor phase by the heating of a ferroelectric capacitor at a temperature higher than the temperature at which the ferroelectric layer is formed. As a result, the amounts of carbon and hydrogen existing in the ferroelectric layer are reduced, whereby fatigue properties and imprint properties of the ferroelectric capacitor are improved.
The second aspect of a process for manufacturing a ferroelectric capacitor comprises the steps of forming a ferroelectric layer on one of a pair of electrodes; forming the other electrode on the ferroelectric layer; heating the ferroelectric layer at a temperature higher than a temperature for forming the ferroelectric layer to control carbon atoms of the ferroelectric layer to be 5xc3x971018 cmxe2x88x923 or less, and forming the other electrode on the ferroelectric layer. According to this process, carbon and hydrogen in the ferroelectric layer after forming a ferroelectric layer are released in vapor phase by the step of heating a ferroelectric layer at a temperature higher than the temperature at which the ferroelectric layer is formed. As a result, the amounts of carbon and hydrogen existing in the ferroelectric layer are reduced, whereby fatigue properties and imprint properties of the ferroelectric capacitor are improved.
The present invention is based on the following observation. In forming a ferroelectric layer comprising PZT by the MOCVD method, Pb(DPM) Zr(DPM)2, Zr(DPM)4, and Ti(iPro)2(DPM)2 are used as raw materials. In the ideal MOCVD method, a bonding between a metal and a precursor is broken near one of a pair of electrodes (the surface of a substrate) and only the metal part is introduced to a ferroelectric layer. However, in practice, the precursor part containing a large amount of carbon and hydrogen may be introduced to the ferroelectric layer, whereby the concentration of impurity in the ferroelectric layer increases. When the ferroelectric layer contains a large amount of impurities, fatigue properties in the ferroelectric memory comprising the ferroelectric layer at the time of repeated rewriting operations tend to deteriorate, and imprint properties at the time of long period data storage also tend to deteriorate. For example, the measurement by the SIMS (secondary electron spectrometry method) results in a hydrogen content of 5xc3x971021 atoms/cm3, carbon of 2xc3x971019 atoms/cm3. When a ferroelectric is inversed repeatedly at the inversion times of 1xc3x97107, ferroelectric properties deteriorate (reduction in charge). Consequently, it is observed that when the concentration of hydrogen and carbon are high, the ferroelectric capacitor does not operate at satisfying level.